The present invention is directed to protective coatings, which may be provided on a wide variety of substrate types, and which are both stable at high temperatures and, in the case of metal substrates, corrosion-resistant. Although a number of materials and procedures for making coatings are known in the art, there remains a need for coatings, which are chemically and physically stable to heat and do not corrode over time. There is a need for better and more durable heat and/or corrosion resistance coatings on various metallic surfaces. For an instance, corrosion resistance coatings that do not degrade by UV radiation (weathering) are highly desired. There is a need for such protective coatings that are also hard and scratch resistant, and bond strongly (preferably chemically bonded) to various substrates. There is also a need for coatings that provide better resistant to corrosive acids.
Further, many prior art techniques involve very long curing times, very high deposition or curing temperatures and/or vacuum processing, limiting ease of manufacturability especially for large parts. Frequently, there are trade-offs between the various desired characteristics of flexibility, hardness, and coating adhesion. In addition, most coatings which exhibit any of the aforementioned properties do not provide “release surfaces,” i.e., they do not repel water and oil based liquids.
Some organometallic, inorganic and fluoroorganic polymers are known for their thermal stability. Certain polysiloxanes, for example, have been the most widely commercialized of these polymers, but there has been nothing to suggest that such polymers would be useful to make high temperature coatings for performance above 250° C., which is already well above the performance of conventional organic based coatings, with the exception of a few polymers that were found to be costly to manufacture and therefore, their usage is limited to low volume and very special applications. Polysiloxanes have typically been used as high temperature oils and elastomers, as well as soft elastomeric coatings used as anti-fouling, waterproofing, and soil-resistance coatings. Organosilicon resins such as T-structured silsesquioxane and MQ resins are used as high-temperature, hard coatings, but their bonding to substrates is known to be problematic and their curing must be performed at relatively high temperature (150 to 300° C.). They are also expensive. The high temperature application of the elastomeric silicones is typically limited to 250° C.
Some polysilsequioxanes are formed “in-situ” by hydrolysis—condensation reactions of RSi(OR′)3, where R is typically methyl, vinyl or phenyl and R′ is methyl, ethyl, propyl, or propyl as reviewed by R. Baney et. al. (Chem. Rev. 95, 1409-1430 (1995)) and for example, reported by M. Furuya et. al., in Silicones in Coatings II, Paper 8 (Conference proceedings, Paints Research Association, Teddington, UK, 1998). The formed resins have no linear structure segments and contain many cages and T units (crosslinking units) with the approximate structure of [RSiO1.5]a[RSi(OH)O]b[RSi(OR)O]c wherein b is typically below 0.1 to prevent undesired crosslinking before processing. In most silsesquioxane cases c is also below 0.1 and a is the predominant feature forming cage structures. These resins have relatively high viscosities as melts or in solutions. They have very limited shelf stability and tend to gel due to their non-linear structure and the presence of acid or base catalyst incorporated to the reaction solution to activate the hydrolysis-condensation reaction.
Polyphosphazenes belong to another category of inorganic polymers with potential stability at temperatures of 400° C. or higher. Again, there has not been any suggestion that such polymers could be used as good high temperature protective coatings; most research efforts have concentrated on the elastomeric, electrical or optical properties of the polymers. These polymers were never commercialized in large scale due to high manufacturing costs and processing difficulties.
Still another family of polymers, which have been used to provide oxidation-resistant and corrosion-resistant coatings, are fluorocarbons such as tetrafluoroethylene, commercially available as TEFLON® and fluoropolyvinyledene. Information concerning tetrafluoroethylene polymers may be found, inter alia, in U.S. Pat. No. 2,230,654 to Plunkett, issued Feb. 4, 1941. Tetrafluoroethylene coatings, however, like coatings prepared from numerous other poly(fluorocarbons), have limited stability at temperatures above about 300° C. When decomposed at high temperature, they can release hazardous HF and oxidized fluorocarbon compounds that are suspected to be carcinogenic. Their bonding to surfaces is very problematic; they tend to be soft and are considerably expensive.
A different approach for protecting metal surfaces against corrosion at high temperatures and in harsh environments is by the application of ceramic coatings. Ceramic coatings can be fabricated at the surface of metals as uniform, hermetically sealed layers that are well-bonded to the substrate. This approach provides another method of protecting metals against chemical attack. However, only thin films can be formed by a single-layer deposition and defects without cracking. Additionally, the equipment, which has typically been necessary to prepare ceramic coatings, is costly, usually consisting of high vacuum chambers which can only process substrates of a limited size, and the process requires deposition times which are often long.
Thick ceramic coatings can be deposited by thermal and plasma spray techniques. However, these coatings have many defects (cracks, pinholes, voids) that reduce their integrity and make such coatings primarily used for thermal barrier purposes.
Preceramic polymers that can be fabricated like organic polymers and then cured and pyrolyzed to give ceramic products are being developed as an alternative for processing advanced ceramics. Very thin ceramic coatings (0.01 to 0.5 μm thick) can be made by simple wet techniques using solutions of organometallic precursors, provided that the substrate is stable at the pyrolysis temperature (400 to 1000° C.). The developed coatings are hard, very stable at high temperatures, and provide protection against corrosion, but only to a certain extent due to their limited thickness. These coatings lack flexibility because their extensive crosslinking network results in a high modulus of elasticity. Thick layers (on the order of 4 μm or greater, more typically in the range of about 10 μm to 50 μm) cannot be obtained by a single deposition operation using preceramic polymers because the coatings tend to crack as a result of the drastic shrinkage that occurs during conversion of the polymer to a ceramic network and, further, because of a mismatch in the expansion coefficient between the coating and substrate. It is possible to limit shrinkage to one dimension, vertical to the surface, only when fabricating relatively thin layers (less than 0.5 μm). Alternatively, the addition of inert and reactive fillers to the coating formulation can be practice to obtain thick coatings, based on concepts that are described, for example, in U.S. Pat. No. 5,635,250.
The following references relate generally to polymeric coatings which are thermally and chemically stable and/or nonwetting, and to polymers which can be used to prepare such coatings.
U.S. Pat. No. 3,944,587 to Katsushima et al. describes certain hydroxypolyfluoroalkyl-containing silane derivatives as water- and oil-repellent agents. The reference states that a variety of material types may be rendered water- and oil-repellent by applying coatings of the disclosed silane derivatives. The silane compounds react with the substrate surface to provide the water- and oil-repellent coatings.
U.S. Pat. No. 3,979,546 to Lewis describes a method for rendering inorganic substrates hydrophobic which involves treating the substrate surface with alkoxy-omega-siloxanols. The siloxanols are prepared by reacting selected cyclic siloxanes with alcohols.
U.S. Pat. No. 4,301,197 to Franz et al. describes the use of selected poly(alkyl hydrogen siloxanes) to treat glass surfaces to improve the release of polymeric materials.
U.S. Pat. No. 4,591,652 to DePasquale et al. describes certain polyhydroxyl silanes or siloxanes as useful in preparing coatings on metal or glass. The coatings are prepared by curing at temperatures in the range of 90° C. to 150° C.
U.S. Pat. No. 4,954,539 to Cavezzan et al. describes thin films of an aqueous silicone emulsion crosslinked by a monochelate of pentacoordinated tin and cured at temperatures in the range of 80° C. to 220° C. The films are stated to be water-repellent and/or nonadhesive.
U.S. Pat. Nos. 4,983,459 and 4,997,684 to Franz et al. describes treatment of a glass surface with a combination of a perfluoroalkyl alkyl silane and a fluorinated olefin telomer to provide a nonreactive, nonwetting surface.
P. Hergenrother, Angew. Chem. Int. Ed. Engl. 29:1262-1268 (1990), generally relates to thermally stable polymers—including polyimides, poly(aryl ethers) and imide/aryl ether copolymers—and their potential uses.
Silicon-containing polymers are described as potentially useful materials in environments which require thermal stability and oxidation-resistance by R. E. Burks, Jr., et al., J. Poly. Sci. 11:319-326 (1973), C. U. Pittman, Jr., et al., J. Poly. Sci. 14:1715-1734 (1976), and P. Dvornic et al., Polymer 24:763-767 (1983).
U.S. Pat. Nos. 5,405,655 and 5,919,572 describe temperature-resistant and/or nonwetting coatings of cured, silicon-containing polymers. The disclosures of these patents describe, for example, substrates having a nonwetting, nonpyrolyzed coating thereon, and methods for providing a thermally stable, non-pyrolyzed coating on a substrate.
U.S. Pat. No. 6,045,873 describes the use of metal flakes in coatings as a method for inhibiting white rust which may be caused by outdoor exposure.
Despite recent advances, there remains a need in the art for adequately adhered, thermally stable, non-pyrolyzed polymeric coatings that are suitable for use in a wide variety of applications. An ideal coating material combines high thermal stability, desirable wetting properties during deposition on various surfaces, good adhesion to substrate, preferably via chemical bonding, resistance to corrosion and chemical degradation such as oxidation, good barrier properties, ease of application, and low cost of production and application. Adequate shelf stability and appropriate duration for applying the formulations (which may be activated to promote curing) as coatings are also critical for practical commercial purposes.
The present invention is directed at addressing one or more of the abovementioned drawbacks, as well as similar issues pertaining to surface coatings.
Selected silicon-containing polymers according to the disclosure can be used to prepare coatings, which overcome some or all of the disadvantages of the prior art and meet some or all of the above-mentioned criteria, i.e., the coatings provided by the method described herein bond well to many substrates and are heat-stable, rapidly cured at relatively low temperatures and in some instances, even at room temperature, display excellent hardness and adhesion, and provide corrosion and/or scratch resistance protection including applications at temperature over 350° C. Additionally, the coatings may be prepared under conditions which render them nonwetting (for waterproofing and soil resistance). Such coatings can be used in their polymeric stage or after pyrolyzing the polymer component to ceramic in high conversion yield.